1. Field of the Invention
The invention relates generally to a method and apparatus for safely operating radiation-emitting well tools. More specifically, the invention relates to methods and apparatus for preventing unintended operation of a controllable radiation source when a well logging tool is not disposed in a wellbore.
2. Background Art
Determining the porosity and fluid content of subsurface earth formations are critical elements in maximizing the profitability of oil and gas (xe2x80x9cformation fluidsxe2x80x9d) exploration. To that end, a variety of techniques have been developed. One of the more well known techniques involves irradiating the subsurface earth formations with high-energy neutrons and monitoring the resulting energy spectra. When neutrons bombard the formations surrounding the wellbore, they induce a radioactive response, generally in the form of neutrons and gamma radiation, which may be recorded by one or more detectors. Depending on the application, either or both types of radiation may be monitored. By using such techniques, it is possible to determine the porosity and fluid content of a given formation, which generally correspond to the amounts of various fluids that may be easily retrieved from a formation.
Various types of radiation sources have been used in well logging systems. For example, neutrons or gamma rays may be generated simply through the use of radioactive isotopes (which naturally decay over time), or an x-ray source may be used. Alternatively, neutrons may be generated through the controlled collision of energized particles in a manner analogous to a fusion reactor. Such a system is commonly referred to as a pulsed neutron generator. When using such a pulsed neutron generator, the formation surrounding the well logging instrument is subjected to repeated, discrete xe2x80x9cburstsxe2x80x9d of neutrons.
One such prior art pulsed neutron generator is described in U.S. Pat. No. 3,461,291 issued to Goodman and assigned to the assignee of the present invention. The neutron source described in the Goodman patent uses an accelerator tube in which charged particles, such as deuterium ions, are accelerated across a potential and contacted with a target element such as tritium. The reaction between the deuterium ions with the tritium target produces a discrete burst of monoenergetic neutrons at an energy level of about 14 MeV. Neutrons are produced (i.e., the neutron source is xe2x80x9cactivexe2x80x9d) when an external power source (which provides the accelerating potential) is activated, and neutrons are not produced (i.e., the neutron source is inactive) when the external power source is deactivated.
A serious concern with radiation sources in general, and pulsed neutron sources in particular, is exposure of personnel to the high energy, radioactive particles produced by the various sources. Depending on the type of radiation source used, a variety of methods to reduce such exposure have been implemented.
Currently, pulsed neutron systems are typically used in well logging tools to make measurements in two different ways. The first, known as xe2x80x9cwireline logging,xe2x80x9d includes recording measurements in a formation of interest after a borehole has been drilled and the drill string (drilling tool assembly) has been removed from the borehole. Wireline logging includes lowering the well logging instrument into the wellbore at one end of an armored electrical cable and withdrawing the instrument while making measurements. There is a delay between the removal of the drill string and the beginning of well logging operations. As a result, the composition of the formation pore space may change, which may mask important data. However, in wireline logging, the instrument operator has total control over the neutron source, so there is little danger of accidental irradiation. Operating power is only applied to the neutron source when activated by the system operator, typically only when the well logging tool is safely below ground in the wellbore. Typically, then, very little radiation is produced when the logging tool is out of the wellbore and electrical power to the source is turned off by the instrument operator.
A second commonly used technique is known as measurement while drilling (xe2x80x9cMWDxe2x80x9d) or logging while drilling (xe2x80x9cLWDxe2x80x9d). In MWD/LWD operations, measurements may be made during the drilling of the wellbore itself. MWD/LWD instrument systems allow acquisition of near-xe2x80x9creal-timexe2x80x9d data on the conditions inside the wellbore. U.S. Pat. No. Re. 36,012, issued to Loomis et al. and assigned to the assignee of the present invention, describes a MWD/LWD apparatus using a pulsed neutron source.
In most cases, however, MWD or LWD tools are not in direct communication with surface-based recording and control instruments, or have a very slow communication up-link. Additionally, down-links are typically rare, and if used, are usually slow. Thus, the electrical power sources which create the acceleration voltage in the controllable neutron source are not under the direct control of the instrument operator. Such lack of direct control can cause safety concerns. As previously explained, pulsed neutron generators and x-ray generators require a separate electrical power source to provide acceleration potential. When pulsed neutron or x-ray generators are used in well logging applications, power for the neutron and/or x-ray generator generally comes from either stored energy devices or via transmission from the surface. Stored energy devices are typically batteries, but may include other devices such as fuel cells. Stored energy devices provide power continuously until they become depleted. In MWD applications, surface power is typically conveyed via drilling fluid (xe2x80x9cmudxe2x80x9d) flow down the drill pipe to a turbine in the MWD tool. In wireline tools, the armored electrical cable is used to provide the power to the electronic source.
When using stored energy devices in a well logging tool, particularly in MWD applications where direct control by the instrument operator is not available, the instrument operator may not be able to determine whether the neutron or x-ray generator is activated or not. In wireline logging or surface-powered MWD systems (such as turbine powered systems), the instrument operator does not have this problem because the operator can stop the power source at the surface. Thus, in wireline or surface-powered MWD systems, the operator maintains direct control over the power source and, thus, maintains control over the production of radiation. With stored energy devices, however, the operator has no such direct control and, thus, the risk of unintentionally exposing personnel to radiation is significantly greater.
What is needed, therefore, are techniques for preventing a controllable radiation source in logging tools, which are not under direct control of the instrument operator, from generating radiation when the logging tools are not in the wellbore.
One aspect of the present invention is a method of operating a well logging tool having a controllable radiation source operatively coupled to a power source. The method includes monitoring at least one downhole condition and sending a disarm signal from at least one interlock to the power source, when the condition in a wellbore changes from a predetermined condition. The disarm signal causes radiation generation to cease.
Another aspect of the present invention is a well logging tool which includes a controllable radiation source, a power source operatively coupled to the radiation source, and at least one interlock operatively connected to the power source.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.